How do I come out to deeply religious parents? My fiance's mother discovered..and may tell my mother. I'm so scared.

You can express your feelings as openly as you can, tell them what it means to be who you are, how you realized you are LGBT, and reassure them that nothing changes in who you are as a person, you're still you though this part of your identity is different than they thought. Make sure they know you want to share it with them because they are important to you, and maybe tell them that you believe in your relationship and you're sure they know that nothing changes in the person you are. You can also encourage them to ask you any questions they have about it, let them express their worries, and explain why the arguments against LGBT people are based on prejudice more than reality. Be patient, it's possible that they'll need more time to come to terms with it, but what truly matters is that they give you a chance to be listened and understood: that's a signal that they're willing to give you a chance. If you're feeling very uncomfortable, you can even consider writing a coming out letter.

That's a difficult decision and I don't envy your position. Personally, I would probably approach it differently depending on how you fit into the LGBTQ+ community (will this affect your relationship with your fiance? is this a gender issue? a sexuality issue?) and whether or not you consider yourself to be a part of the same religion as your parents. There's always going to be a risk in a situation like this and I imagine it's a bit terrifying. However, I think honesty is important. Even if they don't take it well, I imagine they'd take it better coming from you than from your fiance's mother. Some deeply religious people will be open to talking about your sexuality while others won't even consider it. So it's hard to speak directly to your situation. I'd try to do it somewhere where you feel supported (or perhaps with your fiance and/or some friends who already know?) to ease the difficulty of it. Try to be ready for any questions your parents might have (if they're up for talking to you after you tell them). If they try to start a religious debate, explain that you know their stance on the issue, but you and many other members of their religion respectfully disagree on their conclusions (regardless of what religion they are a part of, there are factions of every religion that support the LGBTQ+ community)...the rest will depend on the specifics of your situation. I'm sorry I can't give better advice. In short, I think you just need to find the situation where you'd feel most supported and do your best to come to your parents in love even if they don't reciprocate. Best of luck.